According to Darwin, the higher animals are developed from the lower ones. The lowest form of animal life is protoplasm, which develops into ambæ (cell life), and thence, successively, into synambæ, gastrula, hydra, medusa, worms, hematega, ascidians, fish, amphibians, birds and reptiles, monotremata, marsupials, placental mammals, lemuridæ, monkeys [missing link], man.